1. Field of the invention
This invention relates to a boiler which uses a gaseous fuel and, more particularly, to a boiler suitable for reducing the concentration of nitrogen oxides (NOx) exhausted therefrom when the fuel is combusted.
2. Description of the Related Art
The emission control of NOx which causes photochemical smog becomes more severe each year, and the development of technologies for NOx reduction is actively pursued. The fuel for boilers, which produces less pollutants during combustion, is liquefied natural gas (LNG) and so on. The NOx which is produced when a gaseous fuel with less nitrogen contents is combusted is thermal NOx produced by the oxidation Of nitrogen in the air supplied for combustion in a high-temperature atmosphere. The generation of thermal NOx highly depends on the temperature, and the thermal NOx increases as the flame temperature increases. The flame temperature varies according to the mixing ratio of fuel to combustion air, i.e. the excess air ratio (air quantity/theoretical air quantity), and is highest when the fuel is combusted with an adequate quantity of air (theoretical air quantity), neither excessive nor insufficient, for complete combustion.
In an ordinary gaseous fuel boiler, diffusion combustion is usually performed. In this combustion method, fuel and combustion air are fed through separate nozzles into the furnace and are mixed therein to form a flame thereby providing for flame stability. In this combustion method, however, during the fuel-air mixing process, there invariably exists a zone where the excess air ratio approaches 1 resulting in the flame temperature increasing thereby generating impermissible levels NOx.
The lean combustion, two-stage combustion, and gas recirculation combustion methods have been developed for reducing the amount of NOx by decreasing the flame temperature. The two-stage combustion and the gas recirculation combustion methods are excellent in NOx reduction, but are susceptible to discharging uncombusted gases. In order to prevent this, the furnace has to be large in size, and therefore, these methods are disadvantageous from an economical point of view. The lean combustion is a combustion under a higher excess air ratio. In this method, since the excess air increases, the heat discharged out of the boiler through the combustion gas increases thereby adversely affecting the thermal efficiency of the boiler.
A boiler which employs a premixture flame is disclosed, for example, in Japanese Patent Examined Publication No. 52-26251, which uses a two-stage burning combining a diffusion flame with insufficient air and a premixture flame with excess air. This combustion method is very effective in reducing NOx, but the diffusion flame with an excess air ratio of less than 1 has a long flame form, so that the furnace has to be large. In order to combust uncombusted combustible gases discharged from the air-insufficient diffusion flame, the oxygen in the combustion gas combusted with excess air must be used. To this and, it is necessary to provide sufficient mixing time, so that the boiler has to large. Even when a short premixture flame is employed to enable a compact boiler structure, if combustion is effected under a high excess air ratio, a decrease in boiler efficiency is caused.